The present invention relates to a four-wheel drive vehicle having a front- and rear-wheel engaging mechanism designed to enable differential limiting means provided between the front and rear wheels to be controlled over the range from the direct coupling position to the disengaged position through the slip region by the control of the degree of engagement.
Generally, in the running of automobiles the front wheel drive is superior to the rear wheel drive in straight running stability, but during cornering the front wheel drive meets with the problem that it is relatively difficult to turn the car because force must be applied to the tires through the steering wheel so that the tires are kept from returning to the normal position that is assumed during the straight running. On the other hand the rear wheel drive enables the car to corner relatively easily, but it involves the disadvantage that excessively large drive power may cause the car to turn more than is desired. Accordingly, it is ideal practice from the viewpoint of running of automobiles to drive the front and rear wheels evenly with substantially equal powers, and four-wheel drive vehicles are considerably superior from this point of view.
During cornering, right and left wheels of a car have different radii of rotation. Therefore, in order to absorb the difference and to thereby enable smooth cornering, automobiles are generally provided with mechanisms designed to absorb the difference in number of revolutions between the right and left wheels in accordance with the difference in the radius of rotation, i.e., differential mechanisms (front and rear differential mechanisms). The difference in the radius of rotation also occurs between the front and rear wheels. Therefore, there has been proposed one type of four-wheel drive vehicle which is provided with a mechanism adapted for absorbing the difference in number of revolutions between the front and rear wheels in accordance with the difference in the radius of rotation, i.e., the center differential mechanism.
This center differential mechanism suffers, however, from the following problem. Since the mechanism is adapted to distribute torque evenly to the front and rear wheels, the power transmission limit is balanced with the one of the driving forces acting on the front and rear wheels which has a lower value. For example, if one of the front wheels slips, the driving energy escapes from the slipping wheel, so that the driving force for the rear wheels becomes extremely small. For this reason, a four-wheel drive vehicle with a center differential mechanism may be inferior to a four-wheel drive vehicle with no center differential mechanism in terms of the transmission of drive power when, for example, the vehicle is running on a road surface having a relatively small coefficient of friction. Accordingly, when relatively large drive power is generated during, for example, acceleration, it may be impossible to sufficiently transmit the drive power to the road surface, resulting in the front or rear wheels slipping undesirably.
In order to prevent the occurrence of such an unfavorable phenomenon, a four-wheel drive vehicle with a center differential mechanism has heretofore been provided with a lock mechanism which is adapted to directly couple together the differential limiting means for the front and rear wheels without interposition of the center differential mechanism therebetween, so that, when relatively large drive power is required, for example, when the vehicle is being accelerated or running on a rough road, the center differential mechanism is manually locked, whereas, when the vehicle is in a normal running state wherein no specially large drive power is needed, the center differential mechanism is manually unlocked.
FIG. 10 shows a power transmission mechanism employed in a full-time four-wheel drive vehicle with a center differential mechanism which has an engine mounted on the front side. In this power transmission mechanism, the power from the engine is transmitted to a torque converter 21, a main transmission gear 22 and a subsidiary transmission gear 23, which are disposed within an automatic transmission 20, and the output from the subsidiary transmission gear 23 is transmitted to a driving gear 24 and then to a front-wheel driving shaft 26 through the driving gear 24, thus driving the front wheels. The front differential mechanism 25 provided in this power transmission mechanism is a differential mechanism which acts between the right and left front wheel. On the other hand, a propeller shaft 27 for driving the rear wheels is coupled through a bevel gear 28 to a center differential mechanism 29 which is adapted to act between the front and rear wheels, the center differential mechanism 29 being coupled to a rear-wheel transmission 30. Further, a clutch 31 for locking the center differential mechanism 29 is disposed in parallel to it. Accordingly, the locking of the center differential mechanism 29 is controlled by controlling the engaged state of the clutch 31 by means of an oil-hydraulic circuit (pressure control solenoid) 32.
The above-described power transmission mechanism will be explained in more detail with reference to FIG. 12. The rotation of the engine is transmitted to a front differential case 52 through a ring gear 51 after the speed of rotation has been appropriately changed through an automatic transmission mechanism. In a normal running stage, a clutch 53 for locking a center differential mechanism A is in a disengaged state, and in this state, the rotation of the front differential case 52 is transmitted through a first hollow shaft 55 to a differential carrier 57 in the center differential mechanism A and is further transmitted from a differential pinion 59 to left and right side gears 60 and 61. The rotation of the left side gear 60 is transmitted through a second hollow shaft 62 to a differential carrier 63 in a front differential mechanism B and is further transmitted from a differential pinion 65 to left and right side gears 66 and 67 from which the rotation is transmitted to left and right front wheel driving shafts 69 and 70. On the other hand, the rotation of the right side gear 61 is transmitted to a center differential case 71 which is in spline coupling to the gear 61, and the rotation is further transmitted to a drive pinion shaft 75 through ring gears 72 and 73 for driving the rear wheels and is then transmitted to left and right rear wheel driving shafts (not shown) through propeller shaft and rear differential mechanism (not shown).
When relatively large drive power is required because the vehicle is running on a bad road such as a frozen, sandy or rough road, or when there is a fear of the wheels slipping, the clutch 53 is engaged to lock the center differential mechanism A. In this state the rotation of the front differential case 52 is directly transmitted to the differential carrier 63 in the front differential mechanism B through the clutch 53 and is further transmitted from the differential pinion 65 to the left and right side gears 66 and 67 from which the rotation is transmitted to the left and right front wheel driving shafts 69 and 70. At the same time, the differential carrier 57 and left side gear 60 in the center differential mechanism A which are coupled to the front differential case 52 and the differential carrier 63 through the hollow shafts 55 and 62, respectively, are rotated together in one unit without performing any differential motion, and this rotation is further transmitted to the center differential case 71. Thus, rotation the speed of which is the same as the front wheel driving differential carrier 63 is transmitted to the rear wheel driving ring gear 72, and the right and left rear wheel driving shafts are thereby driven.
In general, four-wheel drive vehicles include full-time four-wheel drive vehicles which are provided with center differential mechanisms as described above, and part-time four-wheel drive vehicles with no center differential mechanisms. In the latter type of four-wheel drive vehicle, either the front or rear wheels are normally driven, and when relatively large drive power is required, for example, when the vehicle is running on a snow-covered road, the other wheels are appropriately coupled directly to the driving shaft through a clutch or the like, thus switching the two- and four-wheel drive modes from one to the other as desired.
One of the conditions which cause the vehicles to slip is large drive power. Therefore, noting the fact that, if the two-wheel drive mode is exchanged for the four-wheel drive mode as described above, the tire gripping force is increased so as to enable prevention of slipping, there has been proposed a technique wherein the two-wheel drive mode is switched to the four-wheel drive mode when the select lever is shifted to a position for relatively large drive power so as to prevent occurrence of slip. More specifically, according to the proposed technique, when the select lever applied to a part-time four-wheel drive vehicle is shifted to the range "1" or "2", the two-wheel drive mode is switched to the four-wheel drive mode. There has also been proposed a technique wherein, when the drive power is relatively large, the two-wheel drive mode is switched to the four-wheel drive mode in accordance with the degree of opening of the accelerator.
However, drive power, particularly, depends strongly on the transmission ratio: namely, the lower the gear position, the larger the drive power. Further, in the case of an automatic transmission, the vehicle driving power varies greatly in accordance with the degree of accelerator opening and the vehicle speed as shown in FIG. 11. Therefore, the occurrence of slip cannot completely be prevented only on the basis of the degree of accelerator opening or the position of the select lever shifted by the manual operation. For example, to rapidly accelerate a vehicle equipped with an automatic transmission, it is common practice to step on the accelerator, and the shift lever is rarely actuated. Therefore, the system in which the two-wheel drive mode is switched to the four-wheel drive mode in accordance with the position of the select lever cannot be expected to function effectively as a slip preventing means.
On the other hand, when the differential operation of the center differential mechanism of a full-time four-wheel drive vehicle is limited, no differential control for the front and rear wheels is performed. Accordingly, when the vehicle turns at low speed, tight corner braking occurs, whereas, when the vehicle is running at high speed, if the front and rear wheels are imbalanced with each other in terms of air pressure or load, the fuel consumption is increased, disadvantageously.